The process for optical fiber production comprises two main stages: one in which a cylindrical preform is made having a radial refractive index profile similar to that of the fibers to be drawn and the other in which the preform is heated and drawn into the fibers.
The first stage requires much care if fibers having good mechanical and light transmissive, properties are to be obtained. More particularly it is necessary to reduce to a minimum metallic and/or water contamination of the preform in order to avoid the high losses due to absorption in the wavelength range used in telecommunications.
To this end, several methods have been developed which, besides ensuring high material purity, allow a reasonable formation rate of the preform.
A widely known method is based on the planar deposition of oxides obtained by chemical synthesis from vapor-phase reactants.
This method named CVD (Chemical Vapor Deposition) has a least two variants: IVPO and OVPO.
The first variant involves deposition on the inner surface of a supporting tube (Inside Vapor Phase Oxidation), while the second variant involves deposition on the outer surface of a supporting mandrel (Outside Vapor Phase Oxidation). In both cases chemical reactants are transmitted towards the support and oxidized by a burner flame in an atmosphere enriched with oxygen to provide powdered silicon dioxide (silica), suitably doped with oxides of other elements according to the refractive index required.
In the case of the IVPO process, the reactants flow into the supporting tube, which is made of silica glass, and the oxidation reactions, with subsequent deposition, are obtained by translation in the direction of the reactant flow of a high-temperature annular zone.
After a suitable number of deposition cycles the preform has a tubular shape and a spongy structure of milky colour or is already vitrified, depending on the temperature at which the previous operations have been carried out. At this point, the preform is collapsed, by application of high temperature, into a solid transparent rod.
These kinds of processes generally present greater flexibility, chiefly in the dopant choice and in the formation of the refractive-index profile, by comparison with outside vapor-phase deposition processes of the OVPO type. In particular, the MCVD process (Modified Chemical Vapor Deposition) derived from IVPO techniques is the most widely used in optical fiber manufacture.
Yet, the inside-vapor deposition processes do not permit high deposition rates and hence high fiber fabrication rates.
In fact they use deposition techniques of the "nonlocal" type, wherein the reaction zone does not coincide with the deposition zone. In the reaction zone glass compounds are synthesized in the form of small particles in a colloidal suspension, while in the deposition zone the glass particles adhere to the quartz substrate as an effect of thermophoretic phenomena.
The deposition quality in terms of uniform dopant concentration, of axial uniformity of deposited mass and of absence of localized imperfections (reactant incorporation into the matrix, diffusion centres, etc.) is strictly dependent upon the flow of the gases carrying the glass particles. Such flow should provide laminar conditions and hence must be limited to a finite range of values which depend on the reactor type.
Furthermore, to avoid the formation of local imperfections in the inside techniques, the deposited material should be vitrified and solidified immediately after the deposition. That implies the use of working temperatures in the deposition zone higher than the temperature of vitreous transition of the deposited matrix. It is clear that if the axial thermal gradient is taken into account a layer of too thick a material cannot be deposited; in fact the external temperature would be locally too high and hence the quartz reactor might be permanently damaged.
The inferior productivity is also due to the fact that at the end of the deposition stage the preform is intended to undergo a collapsing operation on the same apparatus to be used for subsequent cycles.
The external deposition techniques are local deposition techniques. They produce a porous non-solidified structure, consisting of particles at a temperature inferior to that of the vitreous transition of the matrix. The porous structure does not undergo thermal stresses, as it is not yet solidified, thus deposition can be effected intensely and with large dimensions.
After the deposition, the external techniques require a subsequent step for dehydrating and solidifying the structure.
Yet this phase does not entail any shrinking like that due to the collapsing in the case of internal technique, as it can take place in parallel; i.e. it is possible to handle a plurality of performs with the same furnace.
More particularly, it can be noted that typically the MCVD process allows a deposition rate of about 2 g/min, corresponding to a fabrication speed of about 5 km/h. By contrast, the typical processes of external deposition (OVD-outside vapor deposition; VAD-vapor axial deposition), allow a deposition rate of about 4.5 g/min corresponding to a fabrication speed of about 10 kg/h.
Hence the productivity of outside-deposition processes is twice as high as that of inside-deposition processes.
Productivity of inside-deposition processes cannot be easily increased on an industrial scale.
In fact interventions designed to increase deposition rate make the process more critical and hinder reproducibility. E.g. deposition rate may be increased by intervening on the thermal gradient by cooling the supporting tube and/or by initiating the reaction by radio-frequency plasma excitation.
Yet, a massive material deposition is obtained only by a considerable increase in the reactant flow. To keep the flow laminar, it is then necessary to modify the reactor dimensions, by increasing its diameter. Under these conditions also the preform collapsing time is increased in contrast with the initial aims.
In inside processes, in fact, productivity depends on the deposition stage as well as on the collapsing stage. These operations, in fact, are carried out at different times by using the same apparatus.